Squid giant axons will be loaded with Ca by (a) stimulation in 100 Ca seawater or (b) immersion in 3 Ca choline seawater. The extend to which Ca enters will be measured by analysis of axoplasma using atomic abosorption methods. Once values for Ca loading have been established, axons will be unloaded in seawater with normal Ca, low and high Na, and axons treated with metabolic inhibitors will also be studied. The objective of these studies is to characterize the relative dependence of axons on (a) the Na gradient and (b) metabolism in affecting Ca homeostasis.